This ash cloud height graphic is the output of a mathematical model of volcanic ash transport in the atmosphere (PUFF, UAF Geophysical Institute).

This model shows expected movement of an ash cloud in the atmosphere for actual or hypothetical eruptions.

AVO produces this graphic when a volcano is restless by assuming a reasonable hypothetical eruption, in order to provide a pre-eruptive forecast of airspace likely to be affected. During an eruption, AVO will update the forecast with actual observations (eruption start time and duration, plume height) as they become available. See current status for Shishaldin

Colored dots represent the estimated height of the top of the ash cloud, in feet above sea level, as it drifts downwind. [Change the color bar legend to “Height of top of ash cloud”]

This graphic does not show ashfall deposition on the ground; go here for ashfall graphic. Note that it is possible for ash clouds to move overhead with little or no fallout on the ground.